There are several principal modulation methods for electromagnetic signals used in communications. The ones that are most widely used are frequency modulation (FM), amplitude modulation (AM), pulse width modulation (PWM) and phase modulation (PM). There have also been some other less widely used methods for transmitting and receiving information by means of electromagnetic signals. The demands of modern information transfer, in particular computer networking and multi-media communications, have increased the need to transmit more and more information on limited channels of communication. With the ever increasing capacity of digital computers, there is an ever increasing demand for modulation methods to enhance the volume of digital data that can be transmitted and received.
Methods have been developed for increasing the amount of information that can be transmitted and received. One such method is described in U.S. Pat. No. 4,387,455 to Schwartz. This method utilizes several different modulation systems at the same time over the same channel. However, this method uses FM and AM modulation and requires several cycles for each digital bit. Similarly, the device disclosed in U.S. Pat. No. 4,103,238 to Deming, provides for three modulation patterns to be transmitted simultaneously on a single carrier wave. Again, multiple cycles are required for each digital bit. The deficiencies of these methods are typical of efforts to increase the amount of information transmitted.
The method disclosed in U.S. Pat. No. 4,584,692 to Yazuka relies on the same common modulation methods but introduces polarity modulations as a means of enhancing the amount of information that can be transmitted. The polarity of the waves is modulated to encode information and then the original wave and the modulated wave are compared to allow decoding of the information. This results in a modest increase in the amount of information that can be transmitted over a single signal.
Various methods designed specifically for digital information transfer provide some enhancement of the data transfer capabilities. The method disclosed in U.S. Pat. No. 4,001,728 to Schneider is a method of transmitting digital signals through the use of pulse width modulation on an incremental ramp wave. A method of transmitting multiple digital signals on a single carrier wave is disclosed in U.S. Pat. No. 4,347,616 to Murakami. Another method providing for the simultaneous transmission of multiple digital signals independently modulated is disclosed in U.S. Pat. No. 3,805,191 to Kawai.
The method disclosed in U.S. Pat. No. 3,890,620 to Toman provides for the modulation of a carrier wave at prescribed time intervals with digital information. This method, however, points up the limitations of attempts to enhance existing methods of digital information transfer. Incoming digital data must first be stored and then it is recalled for transmission at a rate compatible with the carrier wave modulation. The receiver then extracts the digital information from the signal by synchronization with the transmitter. The resultant signal is subject to interference at both the carrier frequency and the modulation frequency.
U.S. Pat. No. 5,364,536 to Tsujimoto discloses a means of modulating a "data burst" on a carrier signal. Tsujimoto uses a modulation scheme to add a sync burst to the modulated carrier signal. This is accomplished by taking a delayed version of the data burst and the non-delayed data burst and taking the difference. This creates an artificial null in the frequency spectrum of the signal. This null spectrum sync burst is added to the signal before signal transmission. Thus for Tsujimoto, the sync burst becomes a signature burst for identifying the data burst. This method, as with the other known methods, relies upon the modulation of a carrier signal. It also does not provide for allocating time slots to multiple information signals. Also, for Tsujimoto the output digital signal is not calibrated. Instead, the signal strength of the carrier wave is calibrated. The null spectrum sync burst is not used for calibrating the received signal.
The present invention is a method and apparatus for transmitting digital communications. The present invention's primary advantage over traditional modulation techniques is the quantity of digital information that can be transmitted and received. Both FM and AM modulation were developed for transmitting analog signals and, for that reason, are cumbersome in transmitting digital signals. The present invention is designed specifically for transmitting digital signals.
This method does not require a carrier wave to transmit the information. Depending upon the information signal sources and the frequencies utilized, thousands of times more information can be transmitted. In FM systems hundreds and even thousands of cycles are required for just one bit of information. This is also true for AM modulation systems. The present invention provides for the placement of two bytes or more of information in each and every cycle. Another advantage of the present invention is the enhanced signal to noise ratio.
One objective of the present invention is to provide a digital information transfer method which does not require a carrier wave.
Another objective of the present invention is to provide a method and apparatus which substantially increases the amount of digital information that can be transmitted on a single signal.
A further objective is to provide a method and apparatus for transmitting and receiving multiple channels of information on a single communication signal.
A still further objective of the present invention is to provide a method and apparatus for continuously synchronizing a transmitter and receiver so that multiple channels of information can be reliably transmitted on a single communication signal by allocation of time slots to each channel.
A still further objective is to provide a method and apparatus for received signals to be calibrated by the receiver to compensate for signal attenuation, losses, noise, distortion and interference, and thereby to provide for very accurate read out of the digital information transmitted.
A still further objective is to provide a method and apparatus for digital information transfer which can utilize either a common synchronized transmitter or a plurality of remote synchronized transmitters and can utilize either a common receiver or a plurality of receivers.
A still further objective is to provide a method and apparatus for digital information transfer which will increase the signal to noise ratio of the received signals in comparison to other known methods.
A still further objective is to provide a method and apparatus for digital information transfer which will reduce the signal bandwidth required for transmission.
A still further objective is to provide a method and apparatus for digital information transfer which will reduce or eliminate the need for signal filtration by the receiver.
A still further objective is to provide a method and apparatus for digital information transfer which is particularly well suited for modem applications and provides for substantial advancement of modem information transfer rates.